`& {...}` nested class as a mixin – unspecified behaviour

This works as expected:

.foo {
  .bar.baz;
}

.bar {
  &.baz {
    some: stuff;
  }
}

$ lessc this
.foo {
  some: stuff;
}
.bar.baz {
  some: stuff;
}

And that fails:

.foo {
  .bar.baz;
}

.bar {
  & {
    &.baz {
      some: stuff;
    }
  }
}

$ lessc that
NameError: .bar.baz is undefined in .../that on line 2, column 5:
1 .foo {
2     .bar.baz;
3 }

Funnily, it works when the declarations are the other way round:

.bar {
  & {
    &.baz {
      some: stuff;
    }
  }
}

.foo {
  .bar.baz;
}

$ lessc the-other-way-round
.bar.baz {
  some: stuff;
}
.foo {
  some: stuff;
}

I'm using lessc 2.4.0 (Less Compiler) [JavaScript].

Start by reproducing the three examples with lessc 2.4.0 and compare the declaration-order results. Determine the intended behavior for nested & selectors used as mixins, then use that decision to define the expected output or error for each example.